Clarifications:
1. You can build your own firearm.
2. You don't need any forms, unless building something special (e.g. NFA item).
3. You only need to do the last 20% of the work actually, which is why you can buy 80% receivers and finish them yourself.
4. The tools don't need to be owned by you. You have to operate the tools.
5. You cannot build it with intent to sell, but that doesn't mean you cannot sell it later on.
6. There is no limit on how many you can build a year, but if you build too many, you risk the ATF coming down on you.
7. Firearm doesn't need a serial number.
*state law may make any of these points invalid.
Topic has come up a few times, and there is a lot of info on the web about doing it yourself.
This guy has built a lot of stuff.
http://www.cncguns.com/
Should I make with one that uses actual bullets or one that just fires on blackpowder and mini balls for simplicity's sake ?
Depends on your skill level, what you want, what tools you have, etc.
Generally if you are going to copy a design, it would be better to buy the firearm, because the tooling will cost more, and you will need time to read up, and you may screw up a few times.
If you have some new design and have the resources, go for it.
If you just like the idea of building your own, go for it.